I have heard that tortoishell cats (tri-coloured) always are female. Is this true?

Yes, (nearly) all tortoiseshell cats are female. All coat colours except white are bound genetically to the X chromosone. This means that you need 2 X chromosomes to get 2 different colours (other than white) in a cat. Only females have 2 X chromosomes. Males have an X and a Y chromosome. White areas lack colour and are controlled by other genes.

In very rare cases you can get tri-coloured male cats. This is due to a chromosomal defect, for example, that the cat has 2 X chromosome and one Y chromosome (i.e. is a hermaphrodite).